The present invention is directed to automotive electrical systems and, more particularly, to a system for controllably implementing a motoring mode and a generating mode.
Electric generators used in automotive vehicles include multiphase alternators that generate alternating current (AC) electric power when a rotor of the alternator is mechanically rotated, such as by a belt coupled to a motor of the vehicle. A rectifier bridge is typically used to convert the AC electric power output to direct current (DC) electric power for charging one or more vehicle batteries and for powering DC loads. Commonly used rectifier bridges for automotive alternators are formed with diodes and/or metal-oxide-semiconductor field-effect transistors (MOSFETs) and allow for current flow in one direction only. The current flow in a diode is determined by the voltage differential between the anode and cathode, such that when the anode voltage overcomes the forward diode drop and any voltage present on the cathode, a current will flow through the diode. Since the current flow in a diode is based only on voltage differentials, no external controls or circuitry are required for determining when the current should flow. Accordingly, a given rectifier diode bridge configuration may easily be adapted for a chosen phase configuration. When the diodes of a rectifier bridge are replaced with MOSFETs, the efficiency of the rectification is significantly increased because electrical power losses of MOSFETs are much less than those of diodes.
An automotive alternator-starter is typically configured to operate in a generating mode for charging one or more batteries, and to operate in a motoring mode. The rotor of the alternator-starter is mechanically connected to the internal combustion engine (ICE), whereby mechanical rotational power is transferred from the ICE to the rotor in generating mode and from the rotor to the ICE in the motoring mode. The mechanical connection may include a direct connection, a pulley, a belt, and/or other mechanism(s) such as gears, clutch assemblies, etc. In the generating mode, rotation of the ICE causes the rotor of the alternator-starter to rotate, thereby generating a battery charging current. In the motoring mode, the alternator-starter operates as a high torque motor for starting an ICE and/or for use at a lower torque as an auxiliary motor, such as for driving an air conditioning compressor, for preventing the ICE from stalling, and/or for powering the vehicle over short distances. Modern automotive alternators are generally required to supply ever-greater amounts of electrical current. For example, hybrid vehicles may use electricity instead of internal combustion for driving the wheels. Other electrical loading from air conditioning, electric power steering, and other vehicle systems further increases the required alternator electrical capacity.
An automotive alternator-starter is typically an electric machine having a multiple-phase stator winding that acts as an armature in alternator/generating mode. The stator may receive an AC voltage from an inverter when operating in a motoring mode. The electric machine has a rotor that may be in electrical communication with a DC power source in a traditional alternator design that utilizes excitation windings, brushes, and a commutator. In various forms, the electric machine may be brushless and may include permanent magnets. An alternator-starter typically is operated with one or more sensors, a voltage regulator, and a controller. For example, a battery state of charge, temperature, and/or voltage may be monitored for adjusting a charging current being output in a generating mode, for determining whether sufficient battery power is available for an auxiliary motoring function, and for other operations.
In electrical automotive applications such as those implemented in hybrid vehicles, an alternator-starter may be integrated into battery charging that includes regenerative braking, solar panels, plug-in and/or inductive powering of a separate battery charger, and other devices. An alternator-starter may operate in a so-called start-stop mode. For example, a hybrid automotive vehicle's ICE may be turned off during an ‘idle’ event such as when the vehicle is stopped at a traffic light, and then the alternator-starter is required to quickly and efficiently re-start the ICE after the idle event is over. In particular, mechanically driving the gears that cold start or re-start the ICE requires that the alternator-starter operate in motoring mode with a high torque at low rotation speeds. In order to increase the torque for a starting event, a conventional alternator-starter might be formed with a reduced number of stator turns, but this is typically undesirable for the voltage generation in generating mode. Alternatively, a conventional alternator-starter may increase voltage excitation of the rotor, but this is also undesirable for voltage generation in the generating mode. Another conventional alternator-starter has utilized two separate batteries, where the inverter is connected by a MOSFET switch to the positive terminal of a second battery in motoring mode, and where the rectifier bridge is connected by another MOSFET switch to the positive terminal of the main battery when the alternator-starter is operating in alternator mode. However, conventional alternator-starters are not optimized for both start-stop and charging operations.